preserved foods is accountable for a great proportion of the cases
of food poisoning. The preparation of canned foods involves the
application of scientific principles, and since this work is done by
men wholly ignorant of science it is quite remarkable that harmful
effects do not manifest themselves more frequently than they do. Every
can of food which is not thoroughly sterilized may become a source of
danger to health and even to life. It may be of interest for us to
study briefly the methods ordinarily resorted to in the preparation
of canned foods. With most substances the food is cooked before being
put into the can. This is especially true of meats of various kinds.
Thorough cooking necessarily leads to the complete sterilization of
the food; but after this, it must be transferred to the can, and the
can must be properly closed. With the handling necessary in canning
the food, germs are likely to be introduced. Moreover, it is possible
that the preliminary cooking is not thoroughly done and complete
sterilization is not reached. The empty can should be sterilized. If
one wishes to understand the _modus operandi_ of canning foods, let him
take up a round can of any fruit, vegetable, or meat and examine the
bottom of the can, which is in reality the top during the process of
canning and until the label is put on. The food is introduced through
the circular opening in this end, now closed by a piece which can be
seen to be soldered on. After the food has been introduced through this
opening the can and contents are heated either in a water bath or by
means of steam. The opening through which the food was introduced is
now closed by a circular cap of suitable size, which is soldered in
position.
This cap has near its center a "prick-hole" through which the steam
continues to escape. This "prick-hole" is then closed with solder, and
the closed can again heated in the water bath or with steam. If the
can "blows" (if the ends of the can become convex) during this last
heating the "prick-hole" is again punctured and the heated air allowed
to escape, after which the "prick-hole" is again closed. Cans thus
prepared should be allowed to stand in a warm chamber for four or five
days. If the contents have not been thoroughly sterilized gases will
be evolved during this time, or the can will "blow" and the contents
should be discarded. Unscrupulous manufacturers take cans which have
"blown," prick them to allow the escape of the contained gases, and
then resterilize the cans with their contents, close them again, and
put them on the market. These "blowholes" may be made in either end of
the can, or they may be made in the sides of the can, where they are
subsequently covered with the label. Of course, it does not necessarily
follow that if a can has "blown" and been subsequently resterilized its
contents will prove poisonous, but it is not safe to eat the contents
of such cans. Reputable manufacturers discard all "blown" cans.
Nearly all canned jellies sold in this country are made from apples.
The apples are boiled with a preparation sold under the trade
name "tartarine." This consists of either dilute hydrochloric or
sulphuric acid. Samples examined by the writer have invariably been
found to consist of dilute hydrochloric acid. The jelly thus formed
by the action of the dilute acid upon the apple is converted into
quince, pear, pineapple, or any other fruit that the pleasure of the
manufacturer may choose by the addition of artificial flavoring agents.
There is no reason for believing that the jellies thus prepared are
harmful to health.
Canned fruits occasionally contain salicylic acid in some form. There
has been considerable discussion among sanitarians as to whether or
not the use of this preservative is admissible. Serious poisoning with
canned fruits is very rare. However, there can be but little doubt that
many minor digestive disturbances are caused by acids formed in these
foods. There has been much apprehension concerning the possibility of
poisoning resulting from the soluble salts of tin formed by the action
of fruit acids upon the can. The writer believes that anxiety on this
point is unnecessary, and he has failed to find any positive evidence
of poisoning resulting from this cause.
There are two kinds of condensed milk sold in cans. These are known as
condensed milk "with" and "without" sugar. In the preparation of the
first-mentioned kind a large amount of cane sugar is added to condensed
milk, and this acting as a preservative renders the preparation and
successful handling of this article of food comparatively easy. On
the other hand, condensed milk to which sugar has not been added is
very liable to decomposition, and great care must be used in its
preparation. The writer has seen several cases of severe poisoning that
have resulted from decomposed canned milk. Any of the galactotoxicons
(milk poisons) may be formed in this milk. In these instances the cans
were "blown," both ends being convex.
One of the most important sanitary questions in which we are concerned
to-day is that pertaining to the subject of canned meats. It is
undoubtedly true that unscrupulous manufacturers are putting upon the
market articles of this kind of food which no decent man knowingly
would eat, and which are undoubtedly harmful to all.
The knowledge gained by investigations in chemical and bacteriological
science have enabled the unscrupulous to take putrid liver and other
disgusting substances and present them in such a form that the most
fastidious palate would not recognize their origin. In this way the
flesh from diseased animals and that which has undergone putrefactive
changes may be doctored up and sold as reputable articles of diet.
The writer does not believe that this practice is largely resorted
to in this country, but that questionable preservatives have been
used to some extent has been amply demonstrated by the testimony of
the manufacturers of these articles themselves, given before the
Senate committee now investigating the question of food and food
adulterations. It is certainly true that most of the adulterations
used in our foods are not injurious to health, but are fraudulent in a
pecuniary sense; but when the flesh of diseased animals and substances
which have undergone putrefactive decomposition can be doctored up and
preserved by the addition of such agents as formaldehyde, it is time
that the public should demand some restrictive measures.
WIRELESS TELEGRAPHY.
BY PROF. JOHN TROWBRIDGE,
DIRECTOR OF JEFFERSON PHYSICAL LABORATORY, HARVARD UNIVERSITY.
I never visit the historical collection of physical apparatus in the
physical laboratory of Harvard University without a sense of wonderment
at the marvelous use that has been made of old and antiquated pieces
of apparatus which were once considered electrical toys. There can
be seen the first batteries, the model of dynamo machines, and the
electric motor. Such a collection is in a way a Westminster Abbey--dead
mechanisms born to new uses and a great future.
There is one simple piece of apparatus in the collection, without which
telephony and wireless telegraphy would be impossible. To my mind it
is the most interesting skeleton there, and if physicists marked the
resting places of their apparatus laid to apparent rest and desuetude,
this merits the highest sounding and most suggestive inscription. It
is called a transformer, and consists merely of two coils of wire
placed near each other. One coil is adapted to receive an electric
current; the other coil, entirely independent of the first, responds
by sympathy, or what is called induction, across the space which
separates the coils. Doubtless if man knew all the capabilities of this
simple apparatus he might talk to China, or receive messages from the
antipodes. He now, by means of it, analyzes the light of distant suns,
and produces the singular X rays which enable him to see through the
human body. By means of it he already communicates his thoughts between
stations thousands of miles apart, and by means of its manifestations I
hope to make this article on wireless telegraphy intelligible. My essay
can be considered a panegyric of this buried form--a history of its new
life and of its unbounded possibilities.
[Illustration: FIG. 1.--Disposition of batteries and coils at the
sending station, showing the arrangement of the vertical wire and the
spark gap.]
For convenience, one of the coils of the transformer is placed inside
the other, and the combination is called a Ruhmkorf coil. It is
represented in the accompanying photograph (Fig. 1), with batteries
attached to the inner coil, while the outer coil is connected to two
balls, between which an electric spark jumps whenever the battery
circuit is broken. In fact, any disturbance in the battery circuit--a
weakening, a strengthening, or a break--provided that the changes are
sudden, produces a corresponding change in the neighboring circuit. One
coil thus responds to the other, in some mysterious way, across the
interval of air which separates them. Usually the coils are placed very
near to each other--in fact, one embraces the other, as shown in the
photograph.
The coils, however, if placed several miles apart, will still respond
to each other if they are made sufficiently large, if they are properly
placed, and if a powerful current is used to excite one coil. Thus,
by simply varying the distance between the coils of wire we can send
messages through the air between stations which are not connected
with a wire. This method, however, does not constitute the system of
wireless telegraphy of Marconi, which it is the object of this paper
to describe. Marconi has succeeded in transmitting messages over forty
miles between points not connected by wires, and he has accomplished
this feat by merely slightly modifying the disposition of the coils,
thus revealing a new possibility of the wondrous transformer. If the
reader will compare the following diagram (Fig. 2) with the photograph
(Fig. 1), he will see how simple the sending apparatus of Marconi is.
[Illustration: FIG. 2.--Diagram of the arrangement of wires and
batteries at the receiving station.]
S is a gap between the ends of one coil, across which an electric spark
is produced whenever the current from the batteries B flowing through
the coil C is broken by an arrangement at D. This break produces an
electrical pulsation in the coil C', which travels up and down the
wire W, which is elevated to a considerable height above the ground.
This pulsation can not be seen by the eye. The wire does not move;
it appears perfectly quiescent and dead, and seems only a wire and
nothing more. At night, under favorable circumstances, one could see a
luminosity on the wire, especially at the end, when messages are being
transmitted, by a powerful battery B.
It is very easy to detect the electric lines which radiate from every
part of such a wire when a spark jumps between the terminals S of
the coil. All that is necessary to do is to pass the wire through a
sensitive film and to develop the film. The accompanying photograph
(Fig. 3) was taken at the top of such a wire, by means of a very
powerful apparatus at my command. When the photograph is examined
with a microscope the arborescent electric lines radiating from the
wire, like the rays of light from a star, exhibit a beautiful fernlike
structure. These lines, however, are not chiefly instrumental in
transmitting the electric pulse across space.
There are other lines, called magnetic lines of force, which emanate
from every portion of the vertical wire W just as ripples spread out
on the surface of placid water when it is disturbed by the fall of a
stone. These magnetic ripples travel in the ether of space, and when
they embrace a neighboring wire or coil they produce similar ripples,
which whirl about the distant wire and produce in some strange way an
electrical current in the wire. These magnetic pulsations can travel
great distances.
[Illustration:
FIG. 2_a_ represents a more complete electrical arrangement of the
receiver circuit. The vertical wire, W', is connected to one wire of
the coherer, L. The other wire of the coherer is led to the ground, G.
The wires in the coherer, L, are separated by fine metallic particles.
B represents a battery. E, an electro-magnet which attracts a piece of
iron, A (armature), and closes a local battery, B, causing a click of
the sounder (electro-magnet), S. The magnetic waves (Fig. 5) embracing
the wire, W', cause a pulsation in this wire which produces an
electrical disturbance in the coherer analogous to that shown in Fig.
3, by means of which an electrical current is enabled to pass through
the electro-magnet, E.]
In the photographs of these magnetic whirls, Fig. 4 is the whirl
produced in the circuit C' by the battery B (Fig. 2), while Fig. 5 is
that produced by electrical sympathy, or as it is called induction,
in a neighboring wire. These photographs were obtained by passing the
circuits through the sensitive films, perpendicularly to the latter,
and then sprinkling very fine iron filings on these surfaces and
exposing them to the light. In order to obtain these photographs a
very powerful electrical current excited the coil C (Fig. 2), and the
neighboring circuit W' (Fig. 5) was placed very near the circuit W.
When the receiving wire is at the distance of several miles from
the sending wire it is impossible to detect by the above method the
magnetic ripples or whirls. We can, however, detect the electrical
currents which these magnetic lines of force cause in the receiving
wire; and this leads me to speak of the discovery of a remarkable
phenomenon which has made Marconi's system of wireless telegraphy
possible. In order that an electrical current may flow through a mass
of particles of a metal, a mass, for instance, of iron filings, it
is necessary either to compress them or to cause a minute spark or
electrical discharge between the particles. Now, it is supposed that
the magnetic whirls, in embracing the distant receiving circuit, cause
these minute sparks, and thus enable the electric current from the
battery B to work a telegraphic sounder or bell M. The metallic filings
are inclosed in a glass tube between wires which lead to the battery,
and the arrangement is called a coherer. It can be made small and
light. Fig. 6 is a representation in full size of one that has been
found to be very sensitive. It consists of two silver wires with a few
iron filings contained in a glass tube between the ends of the wires.
It is necessary that this little tube should be constantly shaken up
in order that after the electrical circuit is made the iron filings
should return to their non-conducting condition, or should cease to
cohere together, and should thus be ready to respond to the following
signal. My colleague, Professor Sabine, has employed a very small
electric motor to cause the glass tube to revolve, and thus to keep the
filings in motion while signals are being received. Fig. 7 shows the
arrangement of the receiving apparatus.
[Illustration: FIG. 3.--Photograph of the electric lines which emanate
from the end of the wire at the sending station, and which are probably
reproduced among the metallic filings of the coherer at the receiving
station.]
The coherer and the motor are shown between two batteries, one of
which drives the motor while the other serves to work the bell or
sounder when the electric wire excites the iron filings. In Fig. 2
this receiving apparatus is shown diagrammatically. B is the battery
which sends a current through the sounder M and the coherer N when the
magnetic whirls coming from the sending wire W embrace the receiving
wire W'.
[Illustration: FIG. 4.--Magnetic whirls about the sending wire.]
The term wireless telegraphy is a misnomer, for without wires the
method would not be possible. The phenomenon is merely an enlargement
of one that we are fully conscious of in the case of telegraph and
telephone circuits, which is termed electro-magnetic induction.
Whenever an electric current suddenly flows or suddenly ceases to
flow along a wire, electrical currents are caused by induction in
neighboring wires. The receiver employed by Marconi is a delicate
spark caused by this induction, which forms a bridge so that an
electric current from the relay battery can pass and influence magnetic
instruments.
Many investigators had succeeded before Marconi in sending telegraphic
messages several miles through the air or ether between two points
not directly connected by wires. Marconi has extended the distance by
employing a much higher electro-motive force at the sending station
and using the feeble inductive effect at a distance to set in action a
local battery.
It is evident that wires are needed at the sending station from every
point of which magnetic and electric waves are sent out, and wires at
the receiving station which embrace, so to speak, these waves in the
manner shown by our photographs. These waves produce minute sparks in
the receiving instrument, which act like a suddenly drawn flood gate in
allowing the current from a local battery to flow through the circuit
in which the spark occurs, and thus produce a click on a telegraphic
instrument.
[Illustration: FIG. 5.--Magnetic whirls about the receiving wire.]
We have said that messages had been sent by what is called wireless
telegraphy before Marconi made his experiments. These messages had
also been sent by induction, signals on one wire being received by a
parallel and distant wire. To Marconi is due the credit of greatly
extending the method by using a vertical wire. The method of using the
coherer to detect electric pulses is not due, however, to Marconi.
It is usually attributed to Branly; it had been employed, however,
by previous observers, among whom is Hughes, the inventor of the
microphone, an instrument analogous in its action to that of the
coherer. In the case of the microphone, the waves from the human voice
shake up the particles of carbon in the microphone transmitter, and
thus cause an electrical current to flow more easily through the minute
contacts of the carbon particles.
The action of the telephone transmitter, which also consists of minute
conducting particles in which a battery terminals are immersed, and
the analogous coherer is microscopic, and there are many theories to
account for their changes of resistance to electrical currents. We can
not, I believe, be far wrong in thinking that the electric force breaks
down the insulating effect of the infinitely thin layers of air between
the particles, and thus allows an electric current to flow. This action
is doubtless of the nature of an electric spark. An electric spark,
in the case of wireless telegraphy, produces magnetic and electric
lines of force in space, these reach out and embrace the circuit
containing the coherer, and produce in turn minute sparks. _Similia
similibus_--one action perfectly corresponds to the other.
The Marconi system, therefore, of what is called wireless telegraphy
is not new in principle, but only new in practical application. It had
been used to show the phenomena of electric waves in lecture rooms.
Marconi extended it from distances of sixty to one hundred feet to
fifty or sixty miles. He did this by lifting the sending-wire spark on
a lofty pole and improving the sensitiveness of the metallic filings
in the glass tube at the receiving station. He adopted a mechanical
arrangement for continually tapping the coherer in order to break up
the minute bridges formed by the cohering action, and thus to prepare
the filings for the next magnetic pulse. The system of wireless
telegraphy is emphatically a spark system strangely analogous to
flash-light signaling, a system in which the human eye with its rods
and cones in the retina acts as the coherer, and the nerve system, the
local battery, making a signal or sensation in the brain.
[Illustration: FIG. 6.--The coherer employed to receive the electric
waves. (One and a third actual size.)]
Let us examine the sending spark a little further. An electric spark
is perhaps the most interesting phenomenon in electricity. What causes
it--how does the air behave toward it--what is it that apparently flows
through the air, sending out light and heat waves as well as magnetic
and electric waves? If we could answer all these questions, we should
know what electricity is. A critical study of the electric spark has
not only its scientific but its practical side. We see the latter side
evidenced by its employment in wireless telegraphy and in the X rays;
for in the latter case we have an electric discharge in a tube from
which the air is removed--a special case of an electric spark. In
order to understand the capabilities of wireless telegraphy we must
turn to the scientific study of the electric spark; for its practical
employment resides largely in its strength, in its frequency in its
position, and in its power to make the air a conductor for electricity.
All these points are involved in wireless telegraphy. How, then, shall
we study the electric spark? The eye sees only an instantaneous flash
following a devious path. It can not tell in what direction a spark
flies (a flash of lightning, for instance), or indeed whether it has
a direction. There is probably no commoner fallacy mankind entertains
than the belief that the direction of lightning, or any electric spark,
can be ascertained by the eye--that is, the direction from the sky
to the earth or from the earth to the sky. I have repeatedly tested
numbers of students in regard to this question, employing sparks four
to six feet in length, taking precautions in regard to the concealment
of the directions in which I charged the poles of the charging
batteries, and I have never found a consensus of opinion in regard to
directions. The ordinary photograph, too, reveals no more than the eye
can see--a brilliant, devious line or a flaming discharge.
[Illustration: FIG. 7.--Arrangement of batteries of motor (to disturb
the coherer) and the sounder by which the messages are received.]
A large storage battery forms the best means of studying electric
sparks, for with it one can run the entire gamut of this
phenomenon--from the flaming discharge which we see in the arc light
on the street to the crackling spark we employ in wireless telegraphy,
and the more powerful discharges of six or more feet in length which
closely resemble lightning discharges. A critical study of this gamut
throws considerable light on the problem of the possibility of secret
wireless telegraphy--a problem which it is most important to solve if
the system is to be made practical; for at present the message spreads
out from the sending spark in great circular ripples in all directions,
and may be received by any one.
[Illustration: FIG. 8.--Photograph of electrical pulses. The interval
between the pulses is one millionth of a second.]
Several methods enable us to transform electrical energy so as to
obtain suitable quick and intense blows on the surrounding medium.
Is it possible that there is some mysterious vibration in the spark
which is instrumental in the effective transmission of electrical
energy across space? If the spark should vibrate or oscillate to and
fro faster than sixteen times a second the human eye could not detect
such oscillations; for an impression remains on the eye one sixteenth
of a second, and subsequent ones separated by intervals shorter than a
sixteenth would mingle together and could not be separated. The only
way to ascertain whether the spark is oscillatory, or whether it is
not one spark, as it appears to the eye, but a number of to-and-fro
impulses, is to photograph it by a rapidly revolving mirror. The
principle is similar to that of the biograph or the vitoscope, in
which the quick to-and-fro motions of the spark are received on a
sensitive film, which is in rapid motion. One terminal of the spark
gap, the positive terminal so called, is always brighter than the
other. Hence, if the sensitive film is moved at right angles to the
path of the discharge, we shall get a row of dots which are the images
of the brighter terminal, and these dots occur alternately first
on one terminal and then on the other, showing that the discharge
oscillates--that is, leaps in one discharge (which seems but one to the
eye) many times in a hundred thousandth of a second. In practice it is
found better to make an image of the spark move across the sensitive
film instead of moving the film. This is accomplished by the same
method that a boy uses in flashing sunlight by means of a mirror. The
faster the mirror moves the faster moves the image of the light. In
this way a speed of a millionth of a second can be attained. In this
case the distance between the dots on the film may be one tenth of
an inch, sufficient to separate them to the eye. The photograph of
electric sparks (Fig. 8) was taken in this manner. The distance between
any two bright spots in the trail of the photographic images represents
the time of the electric oscillation or the time of the magnetic pulse
or wave which is sent out from the spark, and which will cause a
distant circuit to respond by a similar oscillation.
[Illustration: FIG. 9.--Photograph of a pilot spark, which is the
principal factor in the method of wireless telegraphy.]